Chemical sensors are widely used in industrial environments for process control, environmental control, and other applications. As is well known to those having skill in the art, a chemical sensor is a device which monitors the concentration of a given chemical species in a liquid or a gas. Chemical sensors are often required to be highly sensitive, in order to detect small concentrations of the chemical. They are also often required to withstand harsh chemical environments and/or high temperatures which may be present in process control, environmental control or other applications.
One form of chemical sensor is a gas sensor. Schottky diodes are widely used as gas sensors. As is well known to those having skill in the art, a diode exhibits a very low resistance to current flow in one direction and a very high resistance to current flow in the opposite direction, thereby producing current rectification. As is also well known to those having skill in the art, a Schottky diode produces rectification as a result of nonlinear current transport across a metal-semiconductor contact.
For example, a Schottky diode using a catalytic metal contact such as platinum or palladium, has been shown to be an excellent hydrogen gas sensor. In a Schottky diode, the Schottky barrier height decreases when the device is exposed to a hydrogen containing atmosphere. The hydrogen induced changes are typically detected as a modification of the capacitance voltage (C-V) or the current voltage (I-V) characteristics of the diode. See, for example, a publication entitled Use of the Electroreflectance Technique in Pt/GaAs Schottky Barrier Sensor Characterization by Lechuga et al., Sensors and Actuators, Vol. 32, pp. 354-356, 1992.
Diamond is a preferred material for semiconductor devices because it has semiconductor properties that are better than silicon, germanium or gallium arsenide. Diamond provides a higher energy bandgap, a higher breakdown voltage and a higher saturation velocity than these traditional semiconductor materials.
These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using silicon, germanium or gallium arsenide. Silicon is typically not used at temperatures higher than about 200.degree. C. and gallium arsenide is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for silicon (1.12 eV at ambient temperature) and gallium arsenide (1.42 Ev at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 Ev at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. Ill addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments, i.e., diamond is a "radiation-hard" material.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and use of diamond Schottky diode gas sensors. Unfortunately, it has been found that Schottky diodes fabricated from diamond exhibit frequency dependence of their capacitance/voltage characteristic, thereby limiting the usefulness of diamond based Schottky diodes and gas sensors.
The frequency dependent variation of the capacitance/voltage characteristic of diamond based Schottky devices has been widely investigated. See, for example, the publications entitled The C-V Characteristics of Schottky barriers on Laboratory Grown Semiconducting Diamonds by Glover, Solid State Electronics, Vol. 16, pp. 973-983 (1973); and Electrical Characteristics of Schottky Diodes Fabricated Using Plasma Assisted Chemical Vapor Deposited Diamond Films by Gildenblat et al., Applied Physics Letters, Vol. 53, No. 7, pp. 586-588 (1986).
In these investigations, the frequency dependent variation in capacitance/voltage characteristic has been attributed to the presence of deep level states in the diamond band gap, and to the high resistivity of bulk diamond as a result of diamond's unique energy level structure. Accordingly, characterizations of Schottky contacts have heretofore assumed that the undesirable frequency dependence of the capacitance/voltage characteristic was as a result of the inherent energy level structure (i.e. the deep level states in the diamond bandgap) and high series resistance of the diamond material itself. This undesirable frequency dependence limits the usefulness of diamond based gas sensors, notwithstanding the advantages of diamond as a material for semiconductor devices, especially in high frequency or fast transient applications.